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Why did the National League and American League enter a baseball war in the early 1900s?

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Barry Bonds of the San Francisco Giants swings his bat after hitting his record-breaking 756th home run, with the catcher, umpire, and a cheering crowd behind him at AT&T Park. — History of baseball Giants legend Barry Bonds hitting his record-setting 756th career home run, August 7, 2007.

The "baseball war" in the early 1900s was a conflict between the established National League and the rising American League (formerly the Western League) over territorial rights and player talent.123

The tension escalated when the American League, led by Ban Johnson, sought to expand its presence into major cities already occupied by the National League. While the National League initially permitted the American League to move teams into Chicago and Cleveland, it refused to grant permission for the American League to place franchises in Baltimore and Washington, D.C.124

In response, the American League declared itself a major league in 1901 and aggressively moved teams into National League territories, including Boston, Philadelphia, and eventually New York City. During this period, the American League also actively recruited many of the National League’s star players.143

This competition concluded with the "peace of 1903," known as the Cincinnati Agreement, which established rules for territorial rights, player transfers, and the creation of the World Series.123